This invention relates to a vacuum apparatus which is of service to the purposes such as drying (inclusive of freeze drying), concentration, distillation, cooling, desolventing and the like in the food preservation and other fine chemistry fields.
This invention, as mentioned above, can be used for various purposes. As the exemplary instance of its use there can be enumerated a vacuum apparatus for use in freeze drying. Therefore, explanation will be made with reference to it hereinafter. However, it is to be understood it goes without saying that this invention should not be limited to this alone.
FIGS. 1 and 2 illustrate the exemplary prior art embodiments of the aforesaid vacuum apparatus for use in freeze drying. First, explanation will be made as to these embodiments. In this connection, it is to be noted that like reference numerals will be attached to the parts common to both embodiments.
In these conventional apparatuses, as is generally known, there are carried out a first step for cooling the material being dried accommodated in the one vacuum chamber and freezing and a second step for condensing the water vapor and/or other solvent vapor (which will be referred to as vapor) to be generated from the material being dried by means of the condenser installed in the other vacuum chamber and recovering. In this case, since a large quantity of vapor is treated by the condenser in the former stage of the second step, a load is almost taken off the condenser in the latter stage thereof.
The vacuum apparatus illustrated in FIG. 1 is arranged to cool the vapor present outside of a condenser 10 by virtue of the heat of evaporation of a refrigerant comprising halocarbon, etc. supplied from a refrigeration unit 11 into said condenser 10, thereby condensing said cooled vapor on the condenser 10.
Reference will be made to the practice of drying operation using this apparatus. In the first step, the material being dried is first put on a plurality of cooling and heating shelves 5 installed in a first vacuum chamber or on a supporting means (not shown) disposed therebetween, and then a valve 3 disposed in a duct 6 is closed. Hereat, a valve 13 is opened which is disposed in a first refrigerant pipe line 15 extending from a first refrigeration unit 11 (which is a usual one including a compressor, an oil separator, a condenser and an inter-cooler in the case of two-stage compression unit, and is also inclusive of the case were it is used in two-stage cascade refrigeration unit), simultaneously a valve 14 disposed in a second refrigerant pipe line 16 is closed, and then the refrigeration unit is put into operation.
Consequently, the refrigerant in the first refrigerant pipe line 15 passes through an expansion valve 17 and circulates in the pipe line 15 as indicated with a piece dotted line arrow to thereby cool the brine or heat carrier in a heat exchanger 7. The thus cooled brine is allowed to circulate in a brine pipe line 18 as indicated with a solid line arrow by virtue of a pump 9, thereby cooling the shelves 5. If needed at this time, it may be possible to put a second refrigeration unit 12 in operation so as to cool the brine in a second heat exchanger 8. In this connection, it is to be noted that reference numeral 21 in the brine pipe line 18 denotes a heater.
The shelves 5 are thus cooled and the material being dried is cooled from the room temperature up to a temperature in the range of from -45.degree. C. to -50.degree. C. for freezing purposes, thereby finishing the first step.
Subsequently, the switchover of the first step to a second step takes place. Prior to this, the valve 13 is closed and the valve 14 is opened for the purpose of stopping the flow of refrigerant in the pipe line 15 and starting the flow thereof in the pipe line 16 as indicated with a two piece dotted line arrow, thereby cooling the vapor condenser 10 up to a temperature in the range of from -50.degree. C. to -55.degree. C. In this connection, it is to be noted that reference numeral 20 in the pipe line 16 denotes an expansion valve.
Hereat, the second step is started through such a manner that the flow of refrigerant in the pipe line 16 is continued while the valve 3 is opened simultaneously with the drive of a vacuum pump 4. The pressures in both vacuum chambers 1 and 2 are thus reduced up to between 0.2 mmHg and 0.02 mmHg.
On the other hand, the temperature of shelves 5 is raised to a suitable temperature by turning the heater 21 on and driving the pump 9, said suitable temperature being different depending on the properties of materials being dried, thereby evaporating the water or other solvent contained in the material being dried, during which the sublimation heat deprived for maintaining the temperature of shelves at a suitable temperature is compensated by means of the heater 21. In case where said suitable temperature is lower than the room temperature it sometimes happens that the sum of the energy of pump 9 necessary for circulating the brine and the heat entering from the outside of the vacuum chamber is over said sublimation heat. It is the refrigeration unit 12 that is utilized at this time for the purpose of cancelling the surplus heat and holding the shelves 5 at a desired temperature.
However, the vacuum apparatus as illustrated in FIG. 1 is perceived to involve four defects as mentioned below:
The first defect consists in that since in the second step of this apparatus the load changes in the refrigerating cycle are so large extending from the high load state in the initial stage of drying where a large quantity of vapor is condensed to the low load state of the succeeding stage of drying and further to the almost no-load state in the final stage of drying, namely the highest load amounts to a several--ten times as large as the lowest load which is in excess of the normal regulation range of means for use in the refrigerating cycle, for instance, such as a thermostatic expansion valve, it is difficult to maintain the refrigerating cycle in the optimum state automatically coping with said load changes, and further that the second step, wherein a condensation temperature of about -60.degree. C. is called for at the time of high load, is liable to accidents or troubles such as hardening of lubricating oil owing to excessive cooling in excess of the allowable limit of the refrigerating cycle taking place in the latter stage, deterioration of the lubricating oil owing to excessive heating of the gas discharged from the refrigeration unit caused by excessive compression ratio, blocking owing to over-closing of the thermostatic expansion valve, wet compression owing to over-opening of said expansion valve and the like.
As is evident from the foregoing, this vacuum apparatus involves various difficulties in achieving necessary controls for avoiding the above mentioned accidents or troubles and is unable to control the temperature of the condenser 10 to a predetermined valve accurately against said variable loads and thereby accurately control the vapor pressure which is a factor exerting influence on the quality and treating time of the material being dried.
And, the above mentioned difficulties are further increased under following circumstances.
In the vapor condenser 10 being less than 0.degree. C. the water vapor is condensed into ice which sticks fast to the condensing portions and constitutes ice build-up. Accordingly, when the water vapor is condensed ununiformly on the condensing surface, the thickness of ice increases only at the ununiformly condensed portions, which is liable to become bottle-neck or cause blocking of the vapor passageway. In the case of a dry expansion type evaporator, meantime, it is necessary that the total refrigerant should almost be vaporized into a suitable superheated gas at the outlet. In this apparatus, however, as seen from the foregoing, the capability and capacity of the condenser naturally deteriorate because the heat transfer between refrigerant and evaporator surface is made worse at the predominant portions of the evaporator surface and consequently the evaporation surface does not act as an effective condensing surface. To cope with this, it is necessary to consider a countermeasure for preventing wet compression by separating or evaporating a part of the refrigerant passing in the liquid form through the condenser on the way of the suction line. However, this countermeasure brings about not only the loss of refrigeration capability but also the necessity of extremely minute control of expansion valves against very wide load variations, thereby increasing the burden of an operator.
In this apparatus, further, a sudden switchover of refrigerant from the heat exchanger 7 to the condenser 10 takes place at the time of switchover of the first step to the second step and consequently the load of the refrigeration unit 11 rapidly changes from a small load of -45.degree. C. through -50.degree. C. at once to a large one covering the temperature range of from the room temperature (temperature of condenser 10) to freezing temperature. In addition, the condenser 10 must be cooled to a temperature of -50.degree. C. through -55.degree. C. in 20 minutes or as before the temperature of shelves 5, which are out of cooling action, is raised up by the heat entering from the outside. Accordingly, this rapid load variations thus caused in the refrigerating cycle are liable to bring about various troubles or accidents.
Still further, the above mentioned switchover operation is unreasonable from the economical viewpoint of energy. At the terminal of the first step where the shelves have been cooled to a temperature of -45.degree. C. through -50.degree. C., the heat exchanger 7 is already prepared for a lower temperature which is substantially necessary for the condenser 10. Nevertheless, this switchover operation is observed to incur double losses such that the thus cooled heat exchanger 7 must be heated by means of the heater 21 from necessity of raising the temperature of shelves 5 in the second step which should be said a surplus burden and on the other hand another non-cooled condenser 10 must be cooled rapidly by means of the refrigeration unit 11. Additionally, there is caused a necessity of using the vacuum pump 4 of very high evacuation capacity, for instance, such as less than 10-20 minutes during which the rising of temperature of the material being dried on the shelves 5 which are already out of cooling action can be prevented and thus attaining a pressure sufficient to rapidly prevent the material being dried from melting.
In addition thereto, the second step of this apparatus, as previously mentioned, can not dispense with the heat exchanger 8 and refrigeration unit 12 from necessity of controlling the temperature of the shelves 5. In order to comply with this requirement, this apparatus is in need of an expensive installation cost and an increased installation area.
It is the vacuum apparatus illustrated in FIG. 2 that has been proposed for the purpose of eliminating various defects inherent in the vacuum apparatus illustrated in FIG. 1. This apparatus is a vacuum apparatus which is designed to circulate through the inside of the condenser 22 the brine cooled by the refrigerant in the heat exchanger 7 and condense the vapor on the surface of the condenser 22.
Hereinafter, this apparatus will be explained mainly with reference to the points different from the apparatus illustrated in FIG. 1 while omitting the explanations on the portions common to both apparatuses.
In order that the shelves 5 may be cooled, valves 23 and 24 are opened, a valve 25 is closed and a refrigeration unit 11 is operated thereby to circulate the brine through a first brine pipe line 26 in the direction of a piece solid line arrow. Thus, the first step terminates at the time when the material being dried has been cooled to a predetermined temperature and frozen.
Successively, prior to the switchover to the second step, valves 25 and 27 are opened, valves 23 and 24 are closed and a pump 28 is driven for circulating the brine through a second brine pipe line 29 in the direction of two piece solid arrow, thereby cooling the vapor condenser 22 to a predetermined temperature.
Then, the vacuum pump 4 is put into operation and the valve 3 is opened so as to practice the second process.
Then, a valve 36 is opened and the pump 9 is driven for circulating another brine into the shelves 5 and the heater 21 through a part of pipe-line 32 and pipe 34 in the direction of the three piece solid arrow. And, when the shelves 5 are allowed to have a suitable temperature by means of the heater 21 after the vacuum chamber 1 has reached a predetermined degree of vacuum, the water vapor (and/or other solvents) in the material being dried is sublimated by the heat of shelves 5 and the resulting vapor is condensed by the vapor condenser 31 and trapped. The first step is thus transferred to the second step.
In order to control the temperature of the shelves 5 at a comparatively low temperature herein, if needed to cool the shelves, there may be employed an automatic control of valves, for instance, such that valves 23 and 24 are opened slightly and further the valve 25 is closed slightly in response to temperature signals, whereby the shelves 5 may be cooled by using a brine obtained by properly mixing the brine of an elevated temperature circulating through the shelves 5 and pump 9 with the brine of a lower temperature circulating through the condenser 22 and pump 28. This can dispense with the heat exchanger 8 and refrigeration unit 12 employed in the apparatus illustrated in FIG. 1.
According to this vacuum apparatus, furthermore, it becomes possible to bring about moderate changes in load as well as control the temperature of condenser 22 to a desired valve in the manner of applying a load into the brine circulation line for the purpose of moderating the excessive low load.
As is evident from the aforegoing, this apparatus surely can facilitate the maintenance of the optimum conditions for the refrigerating cycle and accordingly can reduce the occurrence of aforesaid troubles or accidents in the refrigerating cycle. On the other hand, however, this apparatus is defective in that even when the operation of the refrigeration unit is in order and the heat exchanger 7 is cooled sufficiently, if the troubles take place in respect to the valves or the like of the pump 28 or brine pipe line 29 resulting in stoppage or flow of brine in the condenser 22 or decrease in the flow rate of brine, in a moment the temperature of the condenser 22 rises, the vacuum pressure rises, whereby the material being dried melts in a short time and the intended freeze drying fails.
Still further, this apparatus is defective in that it is utilized in a considerable degree for such applications as condenser temperature in the vicinity of 0.degree. C., but a little for the temperature ranging between -50.degree. C. and -60.degree. C. The reason is that due to the presence of two kinds of refrigeration capability losses to be referred to hereinafter which call for an excessively large-sized refrigeration apparatus and a compelled excessive energy consumption, this apparatus should be said unprofitable economically.
The first refrigeration capability loss is a temperature loss resulting from the heat transfer induced twice between the outer surface of the refrigerant evaporator of the heat exchanger 7 and the inner surface of the vapor condenser 22 by the brine. Accordingly, in order that this vacuum apparatus may obtain the condenser temperature identical with that of the apparatus illustrated in FIG. 1, it is necessary that the evaporation temperature should be lower by 6.degree. C.-8.degree. C. corresponding to said temperature loss. When explaining this taking the case of a two-stage compression refrigeration apparatus, in case where the evaporation temperature is -60.degree. C. and the evaporation temperature is further lowered by 6.degree. C.-8.degree. C., the refrigeration capability is deteriorated into about 70% and accordingly this induces the necessity of installing an about 40% larger scaled plant. And, in order to obtain the same degree of refrigeration capability there is a necessity of increasing the energy for that purpose by about 30%.
The second refrigeration capability loss is caused by a pump 28 for circulating the brine from the heat exchanger 7 into the condenser 22. The refrigeration capability of refrigerant comes from an evaporative heat. So, refrigerant has a large refrigeration capability per Kg. In contrast, since the refrigeration capability of brine comes from a sensible heat, brine has a small refrigeration capability per Kg, i.e. one over several tense of that of refrigerant, the condenser 22, whose temperature is required to be uniform, is in need of a pump having a large flow rate and therefore amount of energy for brine circulation is increased, for instance, several ten percent of the net refrigeration capability to be transmitted will thereby be lost.
Still further, this apparatus is defective in that the temperatures of each of the condenser 22 and heat exchanger 7 at the time of switchover of the brine flow from the pipe line 26 to the pipe line 29 after completion of the first step is the temperature of a mixture of brines in both pipe lines (-25.degree. C. through -30.degree. C.) which brings about rapid changes in the load and vapor temperature in the refrigeration unit 11 and that since the heat capacities of both the condenser 22 and heat exchanger 7 are large, it is extremely difficult to cool them up to a temperature of about -50.degree. C. through -55.degree. C. in a short time of about 20 minutes.